Prepared mustard (containing vinegar and the like) is certainly not kosher for Passover. According to Ashkenazic practice, the mustard plant itself is included in "kitniot" and prohibited on Passover.
For Sephardic Jews, however, is there a brand of plain mustard powder that can be assumed to be free of wheat/barley/spelt/oat/rye and therefore kosher for Passover?